How to place a light inside a clear object
This is the result and the node setup:

Important: set the shader property to this: (alpha blend)

THe box needs to have transparency, so that the light can &quot;come&quot; out. This does the first mix shader. The second one mixes then the material of the cube with your plastic look (i just took a diffuse shader to simplify it)
a even simpler way would be this:

UPDATE for cycles:
works too
of course you have to tweak the diffuse material to your plastic material. I just simplified it.
Instead of a lamp i placed another cube with strong emission as light.